This invention relates to the field of energy radiation devices for selective application of electromagnetic energy in lossy dielectric media such as biological tissue, and more particularly, to an energy radiating element suitable for urethral insertion for treatment of prostatomegaly such as benign prostatic hypertrophy, prostatitis, and prostate malignancy.
Electromagnetic probe antennas are clinically used in the treatment of non-operable cancer tumors, wherein the probe or an array of probes is invasively inserted into the tumor. Upon the application of electromagnetic power of the appropriate frequency, the cancer cells can be heated and sustained at elevated temperatures (hyperthermia), causing the cells to lose their ability to divide. Hyperthermia treatment is synergistic with radiation therapy and chemotherapy.
Another important application of electromagnetic probe antennas is in treatment of prostatic disease, malignant or benign. Prostatic disease is relatively common in men over 50 years of age. More than 90% of all men develop benign prostatic hyperplasia (BPH) by the eighth decade of life. It is the most common cause of urinary obstruction in men, and 10-20% of men will require prostatic surgery at some time in their lives to relieve obstructive symptoms. Presently, the treatment of choice for symptomatic BPH is surgery. Surgery carries risks, however, especially for the elderly population most commonly associated with BPH.
An important non-surgical BPH treatment involves the transurethral insertion of a radiating antenna. Electromagnetic energy, usually at microwave frequencies, is transmitted through the antenna to heat the surrounding prostate tissue. The procedure, non-surgical and suitable for outpatient performance with minimal medication, has significant advantages over traditional surgical treatments: lower cost, more rapid recovery, reduced recurrence of symptoms.
The energy radiation pattern of the transurethral antenna is a primary determinant of the effectiveness of the treatment. The energy radiation pattern determines the portion of the tissue affected, and the temperature elevation achieved therein. Numerous antenna configurations have been proposed; unfortunately, none produce a uniform radiation pattern. Consequently, proposed antenna configurations can not provide the uniform tissue heating desired for effective treatment.
Some existing transurethral applicators use monopole antennas. See, e.g., Sozanski et al., U.S. Pat. No. 5,370,676; Hascoet et al., U.S. Pat. No. 5,234,004; Hascoet et al., U.S. Pat. No. 5,480,417; Hascoet et al., U.S. Pat. No. 5,509,929. Deficiencies in monopole antenna designs include leakage currents from the radiating element to the feed, and a generally capacitive load to the generator because the needed quarter wavelength length exceeds the length of the prostatic urethra available. Additionally, as described below, monopole antennas produce non-uniform radiation patterns.
Another proposed design used a monopole antenna, twisted into a helix to reduce its physical length. See Tumer et al., U.S. Pat. No. 5,344,435. The resulting helical structure radiates from its end, which is undesirable for tissue hyperthermia application. It can also radiates from its sides (broadside radiation). The broadside radiation pattern is non-uniform, however. Also, the resulting helical structure is too large for applications such as prostatic hyperthermia at centimetric wavelengths required by both Federal Communications Commission requirements and tissue penetration and heating characteristics.
Other existing transurethral applicators use dipole antennas. See, e.g., Rudie et al., U.S. Pat. No. 5,300,099; Rudie et al., U.S. Pat. No. 5,545,137. Each arm of the dipole is typically one fourth of the wavelength. One complex dipole design uses a circumferential cut in an outer conductor to feed a dipole resonator, electrically insulated from the outer conductor. See King et al., U.S. Pat. No. 5,369,251. The design also adds capacitive loading to make the dipole electrically longer than its physical length. See King et al., U.S. Pat. No. 5,369,251. The resulting structure still suffers from the non-uniform radiation field pattern shortcomings inherent in dipole antennas, as described below.
Another proposed design used a loop antenna with a choke to prevent current leakage between the feed and the radiating element. See, e.g., Wong et al., U.S. Pat. No. 5,301,687. The circumference of such a loop antenna is usually about a wavelength for efficient operation, making the structure too large for applications such as prostatic hyperthermia.
The utility derived from the electromagnetic structure is the pattern of tissue irradiation. The radiation patterns from a monopole or a dipole are non-uniform. FIG. 1 shows a typical configuration of a catheter 3 disposed in tissue 1 such as the prostate. The energy radiation patterns and consequent tissue heating patterns of various antenna structures along sections transverse 4 to the catheter axis, coincident 6 to the catheter axis, and parallel 5 to the catheter axis are of interest.
FIG. 2(a,b,c) shows the radiation patterns of a typical monopole antenna. The pattern of a monopole is a toroid with a central null. Accordingly, the radiation pattern transverse to the antenna axis comprises twin humps as shown in FIG. 2a. The central null corresponds to the periurethral regions, resulting in minimal heating of the tissues most important to be heated. The radiation pattern along the antenna axis is minimal, corresponding to the monopole's central null, as shown in FIG. 2b. The radiation pattern parallel to the antenna axis exhibits a region of non-zero radiation, corresponding to the region of the tissue penetrated by the monopole antenna's toroid, as shown in FIG. 2c. Accordingly, a doughnut-shaped region of the tissue, at a distance from and symmetric about a specific point along the antenna axis, can be heated by a monopole antenna.
FIG. 3 shows the radiation patterns of a typical dipole antenna. The pattern of a dipole is a swept figure eight with nulls at the center and both ends. Accordingly, the radiation pattern transverse to the dipole axis comprises twin peaks with surrounding nulls, as shown in FIG. 3a. As with the monopole, the central null corresponds to the periurethral regions, resulting in minimal heating of the tissues most important to be heated (the dipole antenna can be reoriented to relocate the null, but the null still leads to reduced heating in part of the periurethral region). The radiation pattern along the antenna axis is minimal, corresponding to the dipole's central null, as shown in FIG. 3b. The radiation pattern parallel to the antenna axis exhibits a region of non-zero radiation, corresponding to the region of the tissue penetrated by the dipole antenna's swept figure eight, as shown in FIG. 3c. Accordingly, a region of the tissue, at a distance from and at a specific point along the antenna axis, can be heated by a dipole antenna.
The non-uniform radiation pattern of these electromagnetic structures relates directly to non-uniformity of heating in the periurethral tissues. Indeed, the field non-uniformity is accentuated in the thermal pattern because the heat is proportional to the square of the electric field distribution in three-dimensions (i.e. the electric field Hermitian). Nonuniform radiation patterns imply nonuniform field effects upon the tissue. Some areas are over treated, some are under treated. The former causes unnecessary injury to normal tissue to increase unwanted side effects such as incontinence whereas the latter leaves a tissue nidus for new growth. Regrowth leads to recrudescence of symptoms, such as urinary obstruction and the need for re-treatment.
There is a need for hyperthermia apparatus incorporating an energy radiating element, suitable for transurethral application, that provides uniform energy transfer to surrounding tissue.